FIG. 1 shows a closed-loop drilling system 10 according to the prior art for controlled pressure drilling. The drilling system 10 has a rotating control device (RCD) 12 from which a drill string 14, a bottom hole assembly (BHA), and a drill bit 18 extend downhole in a wellbore 16 through a formation F. The rotating control device (RCD) 12 atop the BOP contains and diverts annular drilling returns to create the closed loop of incompressible drilling fluid.
The system 10 also includes mud pumps 34, a standpipe (not shown), a mud tank 32, a mud gas separator 30, and various flow lines, as well as other conventional components. In addition to these, the drilling system 10 includes an automated choke manifold 20 that is incorporated into the other components of the system 10.
Finally, a control system 40 of the drilling system 10 is centralized and integrates hardware, software, and applications across the drilling system 10. The centralized control system 40 is used for monitoring, measuring, and controlling parameters in the drilling system 10. As such, the control system 40 can be characterized as a managed pressure drilling (MPD) control system. In this contained environment of closed-loop drilling, minute wellbore influxes or losses are detectable at the surface, and the control system 40 can analyze pressure and flow data to detect kicks, losses, and other events and can alter drilling parameters to control drilling operations in response.
The automated choke manifold 20 manages pressure and flow during drilling and is incorporated into the drilling system 10 downstream from the rotating control device 12 and upstream from the gas separator 30. The manifold 20 has chokes 22, a mass flowmeter 24, pressure sensors (not shown), a local controller (not shown) to control operation of the manifold 20, and a hydraulic power unit (not shown) and/or electric motor to actuate the chokes 22. The control system 40 is communicatively coupled to the manifold 20 and has a control panel with a user interface and processing capabilities to monitor and control the manifold 20.
The mass flowmeter 24 is used in the MPD system 10 to obtain flow rate measurements. During operations, for example, highly precise and accurate flow rate measurements are desired along an extended range of flow encountered during managed pressure drilling. However, the typical mass flowmeter 24 inherently loses accuracy at a low end of the flow measurement scale due to internal losses.
A type of flowmeter with the highest accuracy over the full range of desired flow rates is a Coriolis mass flowmeter. The Coriolis flowmeter is valued for its precision and ability to measure volumetric flow rate, mass flow rate, and fluid density simultaneously. For this reason, the flowmeter 24 of the MPD system 10 tends to use a Coriolis flowmeter rated to the highest expected flow rate.
Unfortunately, there are some disadvantages associated with the Coriolis mass flowmeter 24. For example, the fluid connections of the Coriolis mass flowmeter 24 tend to have a lower pressure rating than the rest of the equipment used in the MPD system 10. Moreover, the Coriolis flowmeter 24 is typically rated for a lower working pressure than the choke manifold 20 of the MPD system 10. In particular, the manifold 20 for the MPD system 10 as in FIG. 1 may typically be rated for up to 10,000-psi pressure. However, even though the flowmeter's pressure rating depends on its size and materials, the Coriolis flowmeter 24 is typically limited to a rating of less than 3,000-psi, and usually about 1,500 to 2,855-psi.
For these reasons, the Coriolis flowmeter 24 must be downstream of the chokes 22 due to this pressure limitation, and pressure relief equipment (not shown) is typically necessary should plugging occur in the flowmeter 24. Additionally, the Coriolis flowmeter 24 may be installed with a bypass valve 25 and pressure sensor (not shown). If a pressure limit of the flowmeter 24 is exceeded, the bypass valve 25 is actuated to bypass flow around the flowmeter 24 so drilling can continue at rates that may exceed the capacity of the flowmeter 24.
In addition to some of the physical limitations, the Coriolis mass flowmeter 24 used in MPD operations has some limitations related to its measurement capabilities. For example, even with the improved range of flow rates, the Coriolis mass flowmeter 24 still has a lower accuracy at the lower range of flow rates.
Additionally, the Coriolis mass flowmeter 24 is limited to taking measurements of fluid with low gas content. When too much gas is mixed with the liquid passing through the flowmeter 24, for example, the measurement error of the flowmeter 24 will increase.
One of the causes of rising gas content within the drilling fluid in MPD operations can be cavitation gas breakout that occurs at the choke 22. Valves, such as those used for the choke 22 to control the flow of fluids, have a certain upstream and downstream pressure ratio at which cavitation is likely to occur. This pressure ratio can be characterized by a cavitation index a, which is defined as follows:
  σ  =                    P        u            -              P        v                            P        u            -              P        d            where:
Pu=Upstream Pressure, psig;
Pv=Vapor pressure for given temperature, psig;
Pd=Downstream Pressure, psig; and
σ=Cavitation Index, dimensionless.
The cavitation index σ can change for a valve or choke while it is partially opening or closing. While a valve is closing and flow rate is constant, for example, the cavitation index σ drops. When the cavitation index σ drops to a certain value, cavitating bubbles from gas breakout form within the fluid as it passes through the valve. The specific value of the cavitation index σ at which cavitation occurs can be empirically determined and plotted for all the positions of the valve's components (e.g., stem or the like). As the cavitation index σ continues to drop below the known cavitating value, the quantity of gas that breaks out of the liquid increases.
For these reasons, when the pressures upstream and downstream of the drilling chokes 22 of the MPD system 10 surpass the threshold of the cavitation index a, portion of the cavitating bubbles can travel along the flow path through the Coriolis flowmeter 24 and can cause additional flow measurement error.
In addition to the simple input-output cavitation index discussed above, critical cavitation index is a value that can characterize the effects of local velocity and pressure gradients through a valve, such as the chokes 22. The critical cavitation index can be characterized as:
      σ    i    =            (              P        -                  p          v                    )                      1        2            ⁢      ρ      ⁢                          ⁢              V        2            
σi critical cavitation index
P static pressure in undisturbed flow
pv vapor pressure
ρ liquid density
V free stream velocity of the liquid
This formula describes some of the primary physics behind cavitation.
Another cause of gas breakout in MPD operations is due to flash evaporation that can occur within or near the Coriolis flowmeter 24. Flash evaporation results from the pressure drop through a flow restriction where the downstream pressure is below vapor pressure and σ<1. Cavitation occurs within a range below some critical cavitation number when σ>1.
Yet another cause of gas breakout in MPD operations can involve flashing that can occur within or near the Coriolis flowmeter 24 when positioned at a higher elevation than the flow exit from the system. Due to the design and layout of some drilling rig operations, for example, there may be difficulty in finding a place for positioning the Coriolis flowmeter 24 at the same elevation or lower than the system's flow exit.
Flashing caused by elevation can be a factor if the drilling mud tank is on the ground level and the flowmeter 24 is located more than 34-ft above the tank. This places around 0-psig at the flowmeter 24 assuming a full, steady stream. Even if the tank is less than 34-ft below the flowmeter 24, the fluid pressure can still drop lower than atmospheric pressure at the flowmeter 24. This makes it easier for small variations, steps, or protrusions within the pipe to cause localized flashing. To prevent flashing issues, manufacturers of Coriolis type flowmeters 24 typically indicate that the system's flow exit should be above the flowmeter 24, which can also keep fluid from draining out of the flowmeter 24 if the flow stops.
In an additional way for gas to enter the flowmeter 24, gas entrained in the fluid can be separated out as the fluid undergoes a pressure drop. For example, entrained gas in oil-based mud can break out during the pressure drop at the choke 22. The gas may not mix back into solution, and the gas bubbles can pass through the flowmeter 24, altering the readings.
One solution to cavitation and gas breakout problems has been to add a valve or orifice downstream of the Coriolis flowmeter 24. In this position, the valve or orifice can reduce the effects of cavitation by adding backpressure within the pipe that extends from the chokes 22 to the flowmeter 24. However, the control valve that has been used is typically controlled manually and is unable to be reliably reset during operations as flow conditions change.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.